This invention relates to a novel polymer made up of repeating units of N,N,N',N'-tetraphenyl-p-phenylenediamine, which possesses electrochromic characteristics and acquires high electroconductivity by doping with an electron acceptor.
It is known that polythiophene and polypyrrole doped with an electron acceptor exhibit high electroconductivity of the order of 10.sup.-3 to 10.sup.-2 S/cm. As conductive resins these polymers will have applications to, for example, electromagnetic wave shields. Also it is known that both polythiophene and polypyrrole are electrochemically oxidizable and reducible, so that polymer coated electrodes produced by coating an electrode substrate with either of these polymers can be used in electrolytic cells. Furthermore, both polythiophene and polypyrrole change their color as they are oxidized and reduced. Use of these polymers as electrochromic materials in electrochromic display devices has been proposed.
Both polythiophene and polypyrrole can be formed by electrolytic polymerization. This is favorable for using these polymers in electrochromic devices. However, the obtained polymers are poor in formability or processability since they are insoluble in ordinary organic solvents and are not meltable. This is a matter of serious inconvenience for practical use of these polymers, particularly as conductive resins.
As to electrochromic characteristics, polythiophene assumes a blue color in its oxidized state and a red color in reduced state, whereas polypyrrole assumes a bluish purple color in its oxidized state and a yellow color in reduced state. That is, either of these electrochromic polymers undergoes single-stage oxidation and reduction and exhibits a change between two distinct colors. It is conceivable to utilize an intermediate color by keeping such an electrochromic polymer in an incompletely oxidized or reduced state with application of a fixed voltage at a medium level, but in practice it is difficult to accurately reproduce such an intermediate color unless the potential at the electrochromic electrode is controlled extremely precisely
In general, electrochromic display devices have two electrode layers which are in an opposite arrangement, at least one of which is transparent, and a film of an electrochromic material is laid on at least one electrode layer and is exposed to an electrolyte. One type of transmissive electrochromic display device uses a combination of two kinds of electrochromic materials one of which takes on a characteristic color in its oxidized state while the other takes on color in its reduced state. That is, the two electrodes are coated with two kinds of electrochromic materials, such as WO.sub.3 and Prussian blue, respectively. In operation of the display device, electrochemical oxidation of, for example, Prussian blue on one electrode is accompanied by reduction of, for example, WO.sub.3 on the opposite electrode. However, the WO.sub.3 film as formed and the Prussian blue film as formed are both in the electrochemically oxidized state. Therefore, it is necessary to accomplish electrochemical reduction of one of the two electrochromic films precedent to actual operation of the display device by using a third or auxiliary electrode which is disposed in a marginal region of the cell of the display device. For disposition of the auxiliary electrode a considerable space is needed additional to the display area. Besides, it often becomes necessary to widen the distance between the two electrochromic electrodes with consideration of the resistance of the electrolyte solution between the auxiliary electrode and the electrochromic electrodes. In the case of a large-sized display device, widening of the distance raises another problem that uniformity of the distance over the whole display area is not easily maintained. Therefore, the thickness of the glass substrates of the electrochromic electrodes needs to be increased despite undesirable increase in size and weight of the display device, and/or relatively large-sized spacers need to be disposed between the opposite electrodes despite obstructiveness of such spacers to the sight. To obviate such inconveniences, use of polythiophene or polytriphenylamine as an alternative electrochromic electrode material has been tried. However, polythiophene is inferior in formability as mentioned above, and polytriphenylamine is unsatisfactory in endurance to repeated oxidation and reduction.
In another type of transmissive electrochromic display devices, the two electrode layers in opposite arrangement are both coated with an electrochromic material which undergoes two-stage oxidation and two-stage reduction and can assume three differently stable states, viz. reduced state, first-stage oxidized state and second-stage oxidized state, exhibiting three different colors in the respective states. Prussian blue is a typical example of such electrochromic materials. In this type of display devices the first-stage oxidation and reduction reactions of, for example, Prussian blue are utilized in one electrode and the second-stage oxidation and reduction reactions in the opposite electrode.
To fully utilize the two-stage reactions of Prussian blue it is necessary to use an electrolyte solution containing a small and precisely controlled amount of water. If the amount of water is too large, endurance of Prussian blue to repeated oxidation and reduction is marred. However, in industrial manufacturing very strict control of the content of water is very troublesome.
In reflective type electrochromic display devices, the electrochromic material on the display electrode may have either a single absorption peak as in the case of Prussian blue, WO.sub.3 or Ni(OH).sub.x or at least two absorption peaks as in the case of phthalocyanine complex or polytriphenylamine. WO.sub.3 assumes a blue color in reduced state and becomes colorless in oxidized state. Prussian blue is colorless in reduced state and assumes a blue color in oxidized state. Ni(OH).sub.x is colorless in reduced state and assumes a gray color in oxidized state. Polytriphenylamine is colorless in reduced state and assumes a brown color in an intermediately oxidized state and a dark blue color in a further oxidized state. Ruthenium diphthalocyanine assumes a blue color in reduced state, a green color in an intermediately oxidized state and an orange color in further oxidized state. That is, a multicolor disply is possible by using a suitable electrochromic material. This is an advantage of electrochromic display devices over other kinds of display devices using liquid crystals or light emitting diodes. However, problems are involved in electrochromic materials useful in reflective and multicolor display devices. For example, polytriphenylamine is unsatisfactory in endurance to repeated oxidation and reduction: it is difficult to stably drive each display device more than 10.sup.4 times. In the case of a phthalocyanine complex, multiple colors are exhibited in the course of single-stage oxidation and reduction. Therefore, the driving circuit becomes complicated and costly in order to utilize the color exhibited at an intermediate potential with good reproducibility.